1. Technical Field
The disclosure is directed to a circuit of an operational amplifier. More particularly, the disclosure is directed to a circuit of an operational amplifier which is capable of reducing an area occupied by devices and may be stably and instantly operated at a preset crossing voltage.
2. Description of Related Art
In order to allow a normal operational amplifier to be operated on the condition that a preset crossing voltage is given, the entire circuitry may be designed to have transistors having the same crossing-voltage endurance level. The transistors having the high crossing-voltage endurance levels often occupy significant area; accordingly, the entire operational amplifier also occupies large area, and the resultant manufacturing costs are relatively high.
A conventional operational amplifier during operation is connected between a high voltage source VDD and a ground voltage source VSS. To ensure that each transistor in use may endure the crossing voltage Vds and that the device properties remain unchanged during the operation of each transistor, all of the transistors use the same kind of devices (i.e., a transistor device A) having a preset bias endurance level. Here, the bias endurance level of the transistor device A is restricted to X volts. Note that the value of X is not necessarily a fixed voltage value; instead, the value of X serves to indicate the devices having the high preset crossing-voltage endurance level from the other devices.
According to the operational principle of the operational amplifier, the crossing voltage of each transistor may not be the same while the operational amplifier is in operation, and not all of the transistors endure the voltage crossing over the high voltage source VDD and the ground voltage source VSS. In this case, if the transistor device A is applied to design the transistors, the excessive area occupied by the transistors may be considered as a waste.
Accordingly, how to reduce the area in use of the operational amplifier and simultaneously guarantee the stable and constant operation of the operational amplifier having the preset crossing-voltage level are taken into account in the field pertinent to circuit design.